The goal of this application is to obtain high-resolution structural and dynamics information for the full-length, trimeric HIV-1 envelope glycoprotein (Env) to dissect the conformational changes that occur in Env during receptor binding and fusion using cutting-edge integrative structural biology approaches. The findings will yield valuable insights into the Env structure, changes in epitope exposure during viral entry, conformational rearrangements that lead to fusion, and stabilization of particular conformational states by broadly neutralizing antibodies that enhance potency and breadth. Specifically, the application proposes: 1) To structurally characterize full-length HIV-1 trimers at high resolution; 2) To create soluble mimetics of Env trimers that contain the membrane proximal external region; and 3) To study the dynamic nature of native Env trimers. Towards this end, this highly integrated program utilizes cutting edge x-ray crystallography, electron microscopy and single molecule Fluorescence Resonance Energy Transfer methods as well as other biophysical techniques to probe the different conformational states of HIV-1 Env that pertain to its function in viral entry and neutralization by the human immune system. These new insights will likely prove invaluable for vaccine design or for design of fusion inhibitors to prevent and treat HIV-1 infection.